A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Conventional lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
A conventional lithographic apparatus includes a position control system configured to control the position of the substrate support. This position control system includes a position measurement system which is configured to measure a position of a number of sensors or sensor targets mounted on the substrate support. On the basis of the measured positioned of the sensor or sensor targets the position of another location of the substrate support, for instance a target portion on a substrate for a patterned beam of radiation, may be determined.
During use of the lithographic apparatus forces may be exerted on the substrate support. For instance, during the expose phase, i.e. during projection of the patterned beam on a target portion of the substrate level, level actuations may be performed to position the upper surface of the substrate in a correct orientation with respect to the lens column. As the stiffness of the substrate support is limited, the level actuations or other accelerations may cause temporary deformations of the substrate support. Such deformations may lead to focus errors and/or offset in overlay.
To reduce the risk of deformations of the substrate support and as a consequence focus errors or overlay offset, it has been proposed to increase the stiffness of the substrate support. However, the increasing demand on accuracy and speed of the positioning of the substrate support, the possibilities to increase the stiffness of the substrate support without encountering further problems, for instance with respect to weight have come to their limits.